Although Mutavault has all creature types while it’s animated, it does not have the changeling keyword ability.

8/1/2008

A noncreature permanent that turns into a creature can attack, and its abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn’t matter how long the permanent has been a creature.

7/1/2013

Examples of creature types include Sliver, Goblin, and Soldier. Creature types appear after the dash on the type line of creatures.